1. Field of the invention
The present invention relates to a liquid crystal display device. Specifically, to a liquid crystal display device with a source driver in which a significant signal delay is not generated, and which has a fast response speed.
2. Description of Related Art
Recently, weight reduction and shape thinning of display devices have been required to conform to the weight reduction and shape thinning of personnel computers, televisions, etc. Therefore, flat panel displays such as liquid crystal display (LCD) devices are being developed to meet these requirements, instead of cathode ray tubes(CRTs).
LCDs are display devices for obtaining a desired image signal by applying an electric field to liquid crystals having an anisotropic dielectric constant placed (i.e., injected) between two substrates and controlling electric field intensity, thereby controlling an amount of light transmitted onto the substrates from an external light source(backlight).
Generally, LCD devices have already widely been used as screen display devices for portable information appliances such as cellular phones, computers and personal digital assistants (PDAs), because they are thinner, lighter and consume less electric power compared with CRTs. Further, LCD devices are commonly used in certain fields, because fewer electromagnetic waves are emitted from the LCD devices than from CRTs.
LCD devices are typically used as display devices in portable flat panel displays, and a thin film transistor-liquid crystal display (TFT-LCD), in which a thin film transistor is used as a switching device, is commonly used in LCD devices.
Generally, LCD devices are categorized according to the method of displaying color images into color filter type LCD devices and field sequential driving type LCD devices.
The color filter type LCD devices display desired images by forming a color filter layer including of three primary colors of red (R), green (G) and blue (B) on one of two substrates and controlling an amount of light transmitted onto the color filter layer. The color filter type LCD device displays desired images by controlling an amount of light transmitted onto the R, G and B color filter layers, thereby combining R, G and B colors when transmitting light irradiated from a single light source onto R, G and B color filter layers.
In an LCD device for displaying images by using the single light source and the three color filter layers, the LCD device requires three times as many pixels as an LCD device for displaying images by using black and white colors because each display point in the device is composed of three unit pixels corresponding to R, G and B regions. Therefore, a technology for delicately fabricating these complex LCD panels is required to obtain images of high resolution. Further, it is inconvenient to fabricate the LCD device, because each color filter layer should be formed on a separate substrate, and consequently the luminance of the LCD device is reduced, because light transmittance of each color filter is low.
The field sequential driving type LCD device obtains full color images by lighting independent light sources of R, G and B colors sequentially and periodically and applying corresponding color signals to respective pixel and synchronizing the lighting cycles of the light sources. Specifically, the field sequential driving type LCD device displays images by sequentially time-share displaying lights of three primary colors of R, G and B that are outputted from R, G and B backlights onto one pixel where the one pixel is not divided into R, G and B unit pixels, thereby creating a persistent image for the eyes.
The field sequential driving type LCD devices are further divided into analog driving type LCD devices and digital driving type LCD devices. The analog driving type LCD device displays gradation in a transmission at a level that corresponds to the gradation voltage applied. This is done by setting a plurality of gradation voltages corresponding to the number of gradations to be displayed and selecting one gradation voltage corresponding to gradation data from the gradation voltages so that a liquid crystal panel is driven by the selected gradation voltage.
On the other hand, the digital driving type LCD device displays a gradation by constantly applying a driving voltage to liquid crystals and controlling an applying time of the driving voltage. According to the digital driving type LCD device, a gradation is displayed by constantly maintaining a driving voltage and timely controlling the voltage applying state and the voltage non-applying state, thereby controlling an amount of light that is transmitted through the liquid crystals.
LCD devices have a drawback of having a narrow viewing angle since light, darkness and color tone change according to the screen viewing direction. Various methods for overcoming this drawback have been suggested.
For example, in order to improve the viewing angle of an LCD device, a method for improving the vertical luminance as much as 30% or more by attaching a prism film to the surface of a light guide plate may be used, thereby improving the straightness of incident light from the backlight of the LCD device. A method for increasing the viewing angle by attaching a negative light compensation plate to the surface of the light guide plate may also be used.
Further, although the in-plane switching mode provides vertical and horizontal viewing angles of 160 degrees which is a wide viewing angle that is almost on the same level with cathode-ray tubes, an improved countermeasure for a lower opening ratio is necessary since the in-plane switching mode has a relatively low opening ratio.
Additionally, many techniques for improving viewing angle of the LCD devices concentrate on providing optically compensated bend (OCB) mode LCD devices, polymer dispersed liquid crystal (PDLC) mode LCD devices and deformed helix ferroelectric (DHF) mode LCD devices using thin film transistors (TFTs). Particularly, the OCB mode LCD devices are currently actively being studied due their benefits of fast response speed and wide viewing angle of liquid crystals.
FIG. 1 is a liquid crystal state diagram for explaining the operation of an ordinary OCB mode LCD device.
Referring to FIG. 1, the initial alignment state of liquid crystals positioned between an upper plate electrode and a lower plate electrode is the homogeneous state, and when a certain voltage is applied to the upper and lower plate electrodes, the liquid crystals operate in an OCB mode after the homogeneous state is converted into the bend state through transient splay and asymmetric splay.
As illustrated in FIG. 1, formed OCB mode liquid crystal cells generally have about 10 to 20 degrees of tilt angle and 4 to 7 μm of thickness, and an alignment film of the liquid crystal cells is rubbed in the same direction. A high voltage is applied to the liquid crystal molecules to form the tilt angle of the liquid crystal molecules at 90 degrees in the center of the liquid crystal layer. A voltage to be applied to the liquid crystal molecules is varied to modulate polarization of light passing through the liquid crystal layer by changing the tilt of the rest of the liquid crystal molecules except the alignment film and the liquid crystal molecules in the center of the liquid crystal layer since the alignment of liquid crystal molecules in the center of the liquid crystal layer is horizontally symmetrical so that a tilt angle of the liquid crystal molecules at a specific voltage or less is zero degrees, and the tilt angle of the liquid crystal molecules at a specific voltage or more is 90 degrees. It generally takes several seconds to arrange the liquid crystal molecules of a central portion of the liquid crystal layer to have a tilt angle of 0 to 90 degrees. A reaction time of the liquid crystal molecules is very fast at about 10 μs since the arrangement is a bending deformation having a highly elastic coefficient without back-flow.
The above described conventional LCD device includes an LCD panel equipped with a plurality of pixels, a source driver, a scan driver and a backlight for driving the LCD panel. Therefore, scan signals are sequentially applied from the scan driver, and a data voltage is synchronized with the scan signals to be applied from the source driver to corresponding pixels so that the transmittance of liquid crystals is changed according to the applied voltage, wherein a light is cast on the LCD panel from the backlight so that a screen image is displayed by emitting the light in a luminance corresponding to the transmittance of the liquid crystals.
FIG. 2 is a block diagram illustrating a source driver of the conventional LCD device. Referring to FIG. 2, a source driver 20 of a conventional LCD device includes a digital to analog converter 21 and an amp/buffer 22. The digital to analog converter 21 outputs the converted voltage value by receiving gradation data for red R, green G and blue B that corresponds to screen display data and converting the gradation data into an analog voltage value. The amp/buffer 22 amplifies the analog voltage value so that the amplified analog voltage value is output to an LCD panel 10.
However, a slew rate is limited in the above mentioned source driver 20 of the conventional LCD device due to technical limitations of the output of the operational amplifier included in the amp/buffer 22. That is, the output of the amp/buffer 22 is amplified with a time delay compared with an expected voltage value corresponding to the analog voltage value that is the input of the amp/buffer 22. Since this phenomenon limits frame frequency of an OCB mode LCD device, the conventional LCD device has a problem that the benefit of fast response speed possessed by the OCB mode LCD device is not sufficiently exhibited.